President Donald Trump announced on Thursday that Secretary of Defense James “Mad Dog” Mattis will leave the administration by the end of February.

The president did not announce who would replace Mattis to lead the Department of Defense, saying only that the new Defense secretary would be “named shortly.”

Shortly after Trump made the announcement, the Associated Press published Mattis’ resignation letter. In it, Mattis hinted that all was not well between him and Trump, writing that “you have a right to a Secretary of Defense whose views are better aligned with yours” when it comes to “treating allies with respect” (Mattis is in favor of this) and dealing with countries like China and Russia, whose “strategic interests are increasingly in tension with ours.”

Mattis had long been seen as one of the few “adults in the room” keeping the Trump administration—and the president himself—relatively stable throughout Trump’s volatile mood swings and manic lurches. By the president’s own admission, however, he was primarily thankful for Mattis’ help “with respect to the purchase of new fighting equipment.” Which is to say, the president liked having a shopping buddy around—until he didn’t.